The present invention relates to an apparatus for separating a suspension of fibrous cellulose pulp into at least two portions in a closed housing having inlet and outlets, said apparatus having a rotor means provided with projections, said rotor means comprising a first rotor part surrounded by a screening means, the first rotor part and the screening means in their axial extension forming a screening zone and being arranged to cooperate with each other during rotation of the first rotor part for screening the suspension in fluidized state.
In order to subject a fibrous cellulose pulp of medium concentration, i.e. about 6-15%, to a screening operation, a screening device has to be used which generates pulsations and shearing forces in the pulp so that it becomes fluidized, i.e. is converted into a light flowing form, thus enabling the fibers to move in relation to each other. A part-flow of finer fibrous material (accept) and a part-flow of courser material (reject) can then be obtained by means of screening. A screening apparatus based on the above principle is described in U.S. Pat. No 4,680,108.
A problem encountered in this known apparatus is that the inject can be thickened in the first section of the screening zone, probably because of the fact that the suspension has not yet been fluidizid in this part, at the same time as increasing quantities of fiber bundles collect. Due to this thickening the screening means in the form of a screening basket will be subjected to pressure forces and increased torque from the rotor during its rotation which, besides an increased power requirement, can result in deformation of the screening basket. The capacity and efficiency of the apparatus are also reduced.